Ecology of the New Zealand Rocky Shore Community - A Resource for NCEA Level 2 Biology Darren Smith
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Ecology of the
New Zealand
Rocky Shore Community
A Resource for NCEA Level 2 Biology
Darren Smith
Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Author: Darren Smith
Publisher: New Zealand Marine Studies Centre
Address: 185 Hatchery Road, Portobello, Dunedin 9014
ISBN: 978-0-473-23177-4 (PDF)
ISBN: 978-0-473-23176-7 (Softcover)
January 2013
FOREWORD
The New Zealand Marine Studies Centre launched a nationwide citizen science project in
Seaweek 2013. Anyone can take part in this project which aims to monitor biodiversity
and changes in the New Zealand seashore over time. There are two ways of taking part in
the project
1. Marine Metre Squared is a simple way for individuals, families, schools and
community groups to monitor the shore.
2. Seashore Survey is aimed at secondary school students to tie in with NCEA standards.
This resource book is intended to assist teachers of NCEA Level 2 biology to carry out
studies of the New Zealand rocky shore.
Darren Smith developed this resource when he was on the Endeavour Teacher
Fellowship, New Zealand Science, Mathematics and Technology Teacher Fellowship
Scheme 2012. This scheme was funded by the Ministry of Science and Innovation and
administered by the Royal Society of New Zealand. Darren Smith was hosted by the New
Zealand Marine Studies Centre, Department of Marine Science, University of Otago.
For further resources and information about the Marine Metre Squared project go to
mm2.net.nz
We welcome your comments and suggestions.
Please email marine-studies@otago.ac.nz or phone 03 479 5826.
CONTENTS
PART 1: Life on the Rocky Shore ................................................................... 1
1.1 Introduction ...................................................................................................... 1
1.2 Biodiversity on the Rocky Intertidal Shore ...................................................... 1
1.3 The Intertidal Zones .......................................................................................... 2
1.4 The Abiotic Environment ................................................................................. 4
1.5 The Biotic Environment .................................................................................... 6
1.6 Zonation ............................................................................................................ 8
1.7 Adaptations to Life in the Intertidal Zone: Coping with Stress ....................... 12
PART 2: Surveying the Rocky Shore ............................................................ 22
2.1 Introduction: Why Study the Rocky Intertidal Shore? ................................... 22
2.2 How to Carry Out the Shore Survey ............................................................... 22
2.3 Equipment List ................................................................................................ 24
2.4 Survey Method ................................................................................................ 24
PART 3: A Brief Guide to Seashore Surveys ................................................ 27
3.1 Introduction ..................................................................................................... 27
3.2 Transects ......................................................................................................... 27
3.3 Quadrats .......................................................................................................... 29
PART 4: The Tides ........................................................................................ 35
BIBLIOGRAPHY ............................................................................................. 40
APPENDIX 1 Survey Data Sheet ................................................................... 41
APPENDIX 2 Internal Assessment Resource ................................................. 44
APPENDIX 3 Glossary ................................................................................... 52
Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
PART 1: LIFE ON THE ROCKY SHORE
1.1 INTRODUCTION
New Zealand has approximately 14,000 km of coastline consisting of rocky shores,
sheltered estuarine mudflats, and beaches of loose, mobile sediments like sand and gravel.
Our coasts experience a regular, dramatic change in sea level caused primarily by tides.
On the north and west coasts of the south island sea levels may fluctuate by up to 4 m
between the high and low tides. On average NZ experiences two high tides and two low
tides per day. The intertidal zone, sometimes called the littoral zone, is the area of the
seashore that is regularly covered by the sea during high tides and exposed to the air
during low tides.
The organisms living between the tides are true marine creatures, yet are uniquely adapted
for life at the interface between the land and the sea. As marine species they need to be
submerged in seawater for at least a part of the tidal cycle to feed, respire, excrete, and
reproduce and disperse their offspring. However, like terrestrial species, they also need to
be able to survive the harsh conditions that go with regular periods of exposure to the air
when they are uncovered by the falling tide; these include wide variations in temperature,
salinity, pH and oxygen availability; being battered by wind and breaking waves; the
desiccating effects of wind, sunlight and high salinity; and predation by terrestrial
animals.
So, when it is such a demanding and hostile environment, why are any species found in
the intertidal zone at all? The answer is that the intertidal zone offers unique opportunities
– ecological niches – for those species that have adapted to withstand the demands of
living in such an extreme, changeable environment.
1.2 BIODIVERSITY ON THE ROCKY INTERTIDAL SHORE
Rocky shores have the greatest biodiversity of any coastal habitat in New Zealand as they
provide many ecological niches. These niches are created by the abundance and variety
of food types in shallow coastal seas, the stability and complexity of the rocky substrate
that creates numerous sheltered microhabitats for organisms to live in or on, and the
widely varying abiotic conditions from the low shore to the upper shore that prevent any
single species from dominating the entire shore.
Shallow, coastal seas surrounding New Zealand are very productive compared to the open
ocean. Nutrients washed from the land and from the decay of seaweeds are plentiful; as
light is not limiting at these shallow depths, photosynthetic algae thrive. Rocky shores
also provide stable platforms for seaweeds like kelps to grow to a large size. These
factors mean that seaweed productivity is high and therefore there is abundant food for
grazing animals like marine snails, chitons and urchins, which in turn can support large
numbers of predators like whelks and seastars.
As sessile animals are permanently attached to surfaces they cannot move to forage or
hunt for food so instead depend on their food being transported to them by water
movement in the sea. Food such as microscopic phytoplankton and zooplankton is
1 
Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
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plentiful in the nutrient-rich coastal seas. This plentiful food supply supports many
different species of filter feeding animals in large numbers on the rocky shore which
extract the plankton from the water when submerged by the tide e.g. mussels, oysters,
bryozoans, seasquirts, anemones and barnacles.
Rock surfaces provide a relatively permanent, stable platform for animals and algae to
attach to or shelter beneath in a dynamic environment exposed to strong water currents
and wave action. The rock provides a stable anchorage for many sessile organisms and
allows them to grow to a large size without being dislodged. With a plentiful supply of
food, their relatively large size means that they can produce many offspring. Rocky
shores generally have a three dimensional structure. This complex habitat provides many
different spatial niches. Crevices between and under rocks and boulders create
microhabitats that provide shelter from the heating and drying effects of the sun and the
wind, protection from breaking waves, and a safe refuge from larger predators.
One of the most significant factors in creating a large number of ecological niches in the
intertidal zone is the widely changing abiotic conditions. No single organism is adapted
to withstand the full, extreme range of abiotic conditions across the entire intertidal zone
and cope with the high levels of predation and competition found in the lower shore;
rather, different species have adapted to withstand the biotic interactions and tolerate a
particular range of abiotic conditions found at a particular shore level: these are the
underlying factors that lead to zonation on the rocky shore.
1.3 THE INTERTIDAL ZONES
There are several main tidal zones recognised on the seashore. These are shown in Figure
1.
Splash zone
MHWS
High
Intertidal zone: Mid
Low
MLWS
Subtidal zone
!"#$%&'(''$%&''()(*&+(,-!,)!'.&'/,0.!1,-.'2'
The splash zone is not a truly tidal zone as it is above the height of the average spring
high tide (mean high water springs, MHWS) and so is not normally submerged except in
exceptional conditions such as perigee, during low pressure weather systems when the
#!
Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
water is not held down as much by atmospheric pressure, or during strong onshore winds
when sea water is pushed onto the coast. Although terrestrial, the splash zone is
influenced by wave action and wind, with surf and salt spray wetting the rock. On very
exposed coasts with large waves and surf, the splash zone can be several metres wide.
The intertidal zone is the area of the shore between MHWS and the height of the average
low tides during large spring tides (mean low water springs, MLWS). It can be
subdivided into the high, mid and low intertidal shore. The high shore is regularly
covered by seawater during the highest part of the tides only and generally spends more
time exposed to the air than submerged by water (Figure 2). The mid shore is exposed to
air and submerged by water for approximately equal periods of about 6 hours per tidal
cycle. The low shore is rarely exposed for more than just a few hours at low tide. The
rise and fall of the tides is the single most important factor influencing life in the intertidal
zone.
The subtidal zone is the region of the shore below MHWS. This part is only ever
uncovered by exceptionally low spring tides. It is a fully marine zone but, being near to
the sea surface, it has a higher light intensity, dissolved oxygen concentration and wave
action than the seafloor in deeper water.
MHWS
Vertical height on the shore
Mid shore
MLWS
0 25 50 75
100
Percentage of time exposed to air
Figure 2 Aerial exposure time changes with vertical height on a tidal shore.
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Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
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The main environmental gradient influencing life in the intertidal zone is the time spent
exposed to air. Aerial exposure time increases with height on the shore as submergence
time decreases. Differences in aerial exposure time cause big changes in environmental
conditions over a few vertical metres as shown in Figure 3, and it is gradients in these
Increasing biodiversity, predation
Splash zone
and suspended food availability
Increasing aerial exposure time
Increasing grazing and predation
and interspecific competition
Increasing dissolved nutrients
Increasing ranges of dissolved
O2 and salinity, temp. and UV
from terrestrial animals
Intertidal zone
Subtidal zone
''''!"#$%&'*''>0.-Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
However, these are average temperatures; it must be remembered that in the temperate
parts of New Zealand in winter it is quite possible for intertidal organisms to experience
low tides with air temperatures of 17°C during the middle of the day and -4°C just 12
hours later – maybe even more with wind chill. As the sea temperature may be 11°C at
the same time, the organisms can experience rapid changes in body temperature as they
are covered and uncovered by the tide.
As temperature increases, the risk of desiccation also increases. High air temperatures
coupled with low humidity and coastal breezes mean that the evaporation rate is high.
1.4.2 Salinity
Salinity is a measure of the amount of dissolved minerals or ‘salts’ in water. The salinity
of the open sea remains fairly constant at between 33 and 35 parts per thousand (one part
per thousand is equivalent to 1 g salt dissolved in 1 L water). However, in the intertidal
zone organisms can experience large, rapid changes in the salinity of their environment
due to changes in the weather and climate. On a hot, breezy day, evaporation of
freshwater increases salinity in tide pools, crevices and in sediments. In contrast, large
amounts of rainfall dilute standing water on the shore, decreasing its salinity.
1.4.3 Dissolved oxygen concentration
Oxygen dissolved in the sea results from photosynthesis of marine producers like
phytoplankton and macroalgae and diffusion between the air and the sea at the water
surface. The oxygen content of the sea is approximately 50 times less than the air at
25°C. As seawater temperature increases, dissolved oxygen concentration decreases. As
standing water in tide pools and crevices warms, the oxygen concentration can decrease
below a point that organisms can obtain enough to meet their respiration by simple
diffusion.
1.4.4 Nutrient availability
Important nutrients for algal growth like iron, magnesium, nitrate and phosphate are
dissolved in seawater. Therefore, they increase in concentration down the shore as
seawater submersion time increases
1.4.5 Wave action
The main stress from the coastal marine environment is the amount of wave action.
Shores that receive a lot of wind-driven waves are known as ‘exposed’. The effect of
breaking waves on the shore is to drive salt water higher up the shore than by the tide
alone. This increases the width of the intertidal zones by regularly wetting a region area
of the shore (Figure 4).
Another effect of wave action is to create a mechanical stress on intertidal organisms.
There is a lot of energy in breaking waves and organisms need to be able to withstand
repeated battering by the waves. A cubic metre of water has a mass of 1000 kg or one
tonne, a large wave will contain several cubic metres of water so will transfer a lot of
energy onto the shore as it breaks.
5 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Upper limit of spray and splash by waves
Splash zone
MHWS
Intertidal
zone
MLWS
Exposed Sheltered
shore shore
Increasing exposure
Figure 4 The effects of exposure on zonation.
1.5 THE BIOTIC ENVIRONMENT
The upper shore is a harsh environment and fewer species are adapted to tolerate the
extreme abiotic conditions here than lower on the shore, therefore interspecific
competition for food and space is lower for those species that can cope with these
conditions. As a result, biodiversity generally increases down the shore. With more
species come more interspecific interactions. Biotic factors in an ecosystem are related to
the living parts of that system and include competition for resources, predation, herbivory,
parasitism and mutualism.
The lower vertical distribution limit for most intertidal species is determined by
interspecific competition, food availability and predation pressure.
1.5.1 Interspecific competition
Competition between different species increases down the shore with increasing
biodiversity. The main resource that sessile organisms compete for on the rocky shore is
living space on the rock. Planktonic larvae and algal spores of sessile organisms need
room to settle on, attach and grow. Most seaweeds and many marine invertebrates are
benthic: they live on the seafloor surface. Many of these organisms have planktonic
larval stages that settle, metamorphose and grow to adulthood on the rocky shore e.g.
seaweeds, barnacles, seasquirts, mussels, oysters, and tubeworms. Therefore space on
rocky surfaces to attach and grow is fiercely competed for by many different species.
Although filter feeders compete for food, plankton abundance is rarely a limiting factor in
coastal seas. Similarly, dissolved nutrients are rarely limiting for algae. Like plants,
algae are photoautotrophic: they obtain their nutrition by photosynthesis. Marine algae
do this in one of two ways: they either drift in the well-lit surface waters as microscopic
phytoplankton, or permanently attach to a solid substrate from which they can grow e.g.
6 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
seaweeds1. Seaweeds tend to dominate the shallow subtidal and low intertidal areas of all
but the most exposed shores. Here they compete with each other and with sessile animals
for attachment space on the rock. As seaweeds can grow they can cover large areas of the
rock. This can exclude sessile filter feeding animals as the seaweeds reduce the water
flow to the animals living beneath them2. However several animals will burrow under or
allow themselves to be overgrown by seaweeds as this may offer them some protection
from predators. As seaweeds grow they also compete with other algal species for light.
A dense algal ‘canopy’ can overgrow other slower-growing or smaller algal species,
reducing the amount of light they receive and therefore their rate of photosynthesis.
1.5.2 Predation
Predation pressure from marine animals increases down the shore. Predatory animals like
whelks, seastars and fish are mobile and as the tide rises these animals move up the shore
to feed. They can potentially move to any region of the intertidal zone, but the amount of
time they can spend feeding decreases with height up the shore as the aerial exposure time
increases. These predators generally do not feed when they are exposed to the air and,
unlike many of their prey species, are not adapted to tolerate long periods of exposure.
Therefore, there is relatively low predation pressure from marine animals in the high
shore due to the limited time they can spend feeding in the upper shore. The high shore
therefore acts as a habitat prey refuge from marine predators for sessile species such as
mussels and barnacles, where the total mortality due to predation is lower than it would be
lower down the shore.
Although the high shore offers a habitat prey refuge from marine predators, shore birds
like gulls, tattlers and oyster catchers do prey on species in the upper shore when the tide
is out. Many species in the high shore show adaptations to reduce the risk of predation
from seabirds. Some animals like periwinkles, chitons and topshells will hide in
inaccessible rocky crevices and amongst seaweed; others like shore crabs bury
themselves in the sediments that often accumulate underneath rocks. Many high shore
sessile species possess structural adaptations to withstand predation from seabirds.
Barnacles and mussels have strong shells that are firmly cemented to the rocks to avoid
being broken open or dislodged by foraging birds; with their relatively small size this
means that they are a poor meal for a relatively large bird and are therefore overlooked by
most predators, as larger birds tend to concentrate on larger prey that yield a bigger
nutritional benefit for the energy cost of their efforts. The idea of a small (or large) size
acting as a deterrent to predators is known as a size prey refuge.
1.5.3 Food availability
Filter-feeding animals can only feed when they are submerged so it stands to reason that
they will be more numerous in the lower shore where they spend longer periods covered
1
Not all intertidal algae are large seaweeds or macroalgae. The surface of the substrate is covered by a thin
living film of bacteria and microalgae. This microscopic community is an important source of food for
grazing primary consumers like limpets and topshells. These ‘biofilms’ are the pioneer communities in an
ecological succession on the rocky substrate.
2
Although it does provide new niches for many organisms so increases biodiversity. The surface of the
seaweed provides microhabitats for some epibionts (epi- = surface; biont = organism) like spiral tube
worms, ascidians, bryozoans and hydroids; small crustaceans, echinoderms, molluscs and even fish find
shelter between and beneath the fronds; and the seaweed itself is an important food source for many grazers.
7 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
by seawater. The number of potential food sources decreases as biodiversity decreases
down the shore (this is particularly true for those animals that graze on large seaweeds
that are generally restricted to the lower shore). There is also the ever-present risk of
desiccation and thermal shock for animals that are active on the exposed shore during low
tide. Hence for most intertidal animals, feeding opportunities are limited to the times and
regions of the shore that are covered by seawater. All of these factors mean that
opportunities to feed decrease with increasing height on the shore.
However, for scavenging detritivore animals like isopods (sea slaters) and amphipods
(sand hoppers) there are more feeding opportunities higher up the shore as plant and
animal remains collect in the drift line, where they are washed up by the high tide and
stranded as the tide ebbs.
1.6 ZONATION
Biological zonation is the distribution of species into visible bands or zones along
(perpendicular to) an environmental gradient.
On the rocky shore, the environmental conditions may change from fully terrestrial to
fully marine over a few vertical metres. As a result of this strong environmental
gradient, intertidal species occur in discrete horizontal zones according to their ability to
cope with the different physical stresses of the gradient (Figure 5), and their interactions
with other species in their habitat (that are also affected by the same abiotic factors).
In the intertidal zone, tolerance for a particular, limiting abiotic factor usually
determines the upper vertical limit of a species’ distribution, whereas the lower limit tends
to be set by biotic factors such as interspecific competition and predation.
Tolerance is the ability of an organism to withstand a range of values for an abiotic factor
in a changing environment e.g. temperature, salinity, pH and light intensity. Tolerance has
a physiological basis: there are biological processes occurring within the organism that
enable it to maintain an internal environment that allows the organism to survive despite
the changing external environment. If the external environment exceeds the tolerance
limits of the organism for too long, then it cannot regulate its internal environment and it
will die. Figure 6 shows how a species might respond to a changing abiotic factor. The
response could be for an individual organism in terms of its growth, respiration,
reproductive rate or for a population.
8 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Barnacle zone
Tubeworm/
Oyster zone
Seaweed zone
Figure 5 Biological zonation on a near vertical 1.5 m rock surface at Portobello,
Dunedin. Here, aerial exposure time increases with height.
An organism will have an upper and a lower tolerance limit for different abiotic factors
and these determine the tolerance range. Organisms cannot survive the extreme
conditions outside this range. An organism will have an optimum range within which it
is best adapted, and its response is the greatest. Either side of the optimum range lay the
physiological stress zones where the organism ‘struggles’ to survive as it uses energy and
nutrients to oppose the negative environmental conditions, leaving less for growth and
reproduction. For example, in a cold environ-ment, an organism may have to actively
warm itself up by moving to a warmer region or, if it is a ‘warm-blooded’ animal3, it may
increase its metabolic rate.
3
‘Warm-blooded’ animals should be referred to as homoeothermic, meaning that they normally maintain a
relatively constant internal body temperature that is independent of the environmental temperature.
Similarly ‘cold-blooded’ animals maintain a body temperature that is the same as their environment, warm
or cold.
9 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Tolerance Range
Lower Optimum Upper
stress zone range stress zone
zone Optimal zone
Lower limit Upper
value
of tolerance limit of
limit zone tolerance
limit zone
Species response
Value of abiotic factor
Figure 6 Typical physiological tolerance curve showing how species respond to changing abiotic
factors that they cannot escape from.
No two species can occupy the same ecological niche indefinitely; interspecific
competition will ultimately result in the exclusion of the poorer-adapted species from that
niche i.e. one species’ population will decline as the others’ grows: this is Gause’s
Principal of Competitive Exclusion.
The potential niche a species could occupy in the absence of better-adapted competitor is
known as its fundamental niche. As interspecific competition is one of the more
important factors determining the lower vertical distribution limit on the shore, this would
seem to suggest that species occupying the higher shore could potentially live further
down. This has been observed in field experiments where scientists removed a species
from the rock and observed the expansion of a competing species’ zone as it populates the
newly cleared area in the lower shore.
Predation on the rocky shore has a similar effect. The high shore may act as a relatively
safe prey refuge, where mobile marine predators like seastars and whelks either cannot
tolerate the harsher environmental conditions during exposure, or have very limited
feeding opportunities due to the shorter periods of submergence in the high shore when
the predators are active.
10 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Evidence from ‘cage exclusion’ experiments has shown that if predators are kept out of
the lower shore using cages over the rock that allow prey species to settle, but are too
small to admit predators, then the prey species are able to settle and populate the lower
shore.
The effects of interspecific competition and predation therefore act to limit the spatial
niche that many rocky shore organisms can occupy; this means that the actual or realised
niche of the species are often smaller than their potential or fundamental niches4.
However, one outcome of this is that many species are able to coexist on the shore and no
single species tends to dominate. To illustrate these concepts, let us look at an example.
Figure 7 represents the distribution of three interacting species that occupy two adjacent
zones on the mid shore.
The barnacles and the oysters compete for living space on the rock. At the end of their
planktonic larval stages both species cement themselves to a bare patch on the rock and
remain there for the rest of their lives. Barnacles and oysters are both filter-feeders. They
have different structural adaptations for extracting their planktonic food from the water.
The barnacle uses feathery modified legs to comb the water for food particles. Oysters
(and other bivalves like mussels) have rows of cilia on their gills that create feeding
currents to draw water and plankton into the gills which are mucus lined to trap food.
Although both species reduce their metabolic rate during exposure to conserve energy and
reduce oxygen demand and waste build-up, the barnacles can open their shell valves to
take some air into the shell during exposure which enables them to continue respiring at
low tide. Barnacles have a higher tolerance to aerial exposure than oysters and can
withstand being uncovered by the tide for longer periods.
The third species is a predatory whelk that feeds on both the barnacles and the oysters
when submerged by the tide. Just before exposure by the falling tide, the whelk will seek
a damp, shaded crevice in or under the rock. It secretes sticky mucus on to the base of its
foot for attachment to the rock surface. A tough, waterproof pad on the foot called an
operculum seals the snail into its shell and it remains inactive until it is resubmerged by
the rising tide.
In the example in Figure 7, the realised spatial niche of the barnacle species has been
determined by the barnacles’ tolerance for aerial exposure, the presence of the competing
rock oysters, and by predation from the lined whelks. In the absence of the oysters and
whelks it is likely that the barnacles’ distribution would increase down the shore.
However, it is unlikely to increase up the shore as the barnacles is likely to be living at its
upper tolerance limit for some limiting abiotic factor associated with the long periods of
aerial exposure. Therefore, the barnacles’ fundamental spatial niche is wider than the one
it is forced to occupy, but only in terms of the lower distribution limit.
4
Interspecific competition for resources other than living space can limit a species’ distribution and
therefore reduce the size of its realised niche e.g. food availability and periods of activity.
11 Ecology of the New Zealand Rocky Shore Community:
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Upper Limit
Actual range
Hunting time decreases
Potential vertical range
Lower Limit
Acorn barnacle Rock oyster ./01&%20'%34#&!
Lined whelk
Austrominius modestus Ostrea chilensis Buccinulum
linea
Figure 7 The effects of tolerance, predation and interspecific competition on the upper and lower
vertical distribution limits of acorn barnacles (Austrominius modestus).
1.7 ADAPTATIONS TO LIFE IN THE INTERTIDAL ZONE:
COPING WITH STRESS
The organisms living in the rocky intertidal zone are almost all marine in origin but have
adapted5 to cope with the demands of spending part of their day uncovered by the sea.
Many of the adaptations that have evolved in intertidal organisms to survive to help them
cope with, avoid or minimise stresses associated with periods of aerial exposure. Of all of
the widely varying abiotic factors, heat and desiccation are probably the most important in
setting the upper limits of intertidal species’ distribution on the rocky shore.
1.7.1 Heat and desiccation
Even in cold, temperate environments like southern New Zealand, the air temperature can
reach over 30°C in summer. However, it is not just the maximum temperature that is
important, but the temperature variation. Seasonal variations over a year can account for
air temperatures between 30°C and -5°C at sea level in Dunedin. Monthly variations may
be equally dramatic: in March 2012 the highest recorded air temperature in Invercargill in
A
!This has occurred over many generations as they have evolved by natural selection.!
"#!Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
southern New Zealand was 24.3°C while the lowest was 1.5°C (Deep South Weather,
2012). Daily variations in air temperature of 20°C between day and night are not
uncommon in New Zealand. However, drastic changes in temperature can occur much
more rapidly if the influences of the tide, time of day and sea temperature are considered
together. For instance, during a midday low tide, an organism’s body temperature can
rise from the temperature of the seawater to well above that of the air temperature, a
difference of up to 25°C (Helmuth, 1998). The organism’s body temperature could then
rapidly drop back down to that of the seawater when it is re-immersed by the rising tide
(see Figure 8), but on a clear night just a few hours later, it can decrease to several
degrees below air temperature during the next low tide.
Immersion by rising tide
Body Temperature
(°C)High
Time of Day (hr)
Figure 8 Changes in body temperature of the intertidal mussel Mytilus
californianus over part of a tidal cycle during the summer. Modified from
Helmuth (1998).
Intertidal organisms are ectothermic which means that they lack any physiological
mechanisms to generate body heat. As a result, their body temperature is greatly affected
by their environment. This is particularly true for small organisms that have a low body
mass and therefore a relatively low heat capacity.
Physiology involves the biochemical reactions and processes that occur within an
organism to maintain life. For every 10°C increase in body temperature, the rate of a
biochemical reaction is approximately doubled, and for every 10°C decrease the rate is
halved. Hence an organism that experiences a temperature change of 20°C would
experience a fourfold change in metabolic rates e.g. their respiration rate would increase
by four times and so their rate of glucose use, oxygen demand and carbon dioxide
production would increase accordingly; a marine organism exposed to the air cannot feed
or carry out gas exchange with seawater, so normal rates of aerobic respiration cannot be
sustained.
High temperatures also disrupt the organisation, stability and structure of delicate
molecules in cells, such as membrane lipids, enzymes, DNA and RNA. For many
important biological molecules, function is dependent on structure, so any change in
structure may cause a loss of function.
13 Ecology of the New Zealand Rocky Shore Community:
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Intertidal organisms can slowly adjust to long-term seasonal variations, but not to very
short-term changes over the course of hours: therefore they need to withstand this short-
term variation by tolerating it, avoiding it or reducing its effects.
Most intertidal organisms live at a level of the shore that is very close to their upper
thermal tolerance limit i.e. close to the point where the high environmental temperatures
would be fatal. But how can some species live higher on the shore than others where they
experience not only higher temperatures, but a greater range of environmental
temperatures?
Many intertidal species are eurythermal: they can tolerate much higher environmental
temperatures than their non-intertidal relatives. Their physiological processes have
adapted to function at higher temperatures. Figure 9 shows how the upper lethal limit of
temperature of five different species of porcelain or half-crab varies with their vertical
position on the shore at each of four different regions in the western Pacific Ocean. The
differences in the upper thermal tolerance limit of the closely related crabs can be related
to both their vertical position on the shore and their climate in relation to their latitude
(distance from the equator).
Intertidal species also have a much wider temperature tolerance range than subtidal
species. In controlled laboratory incubation experiments by Somero (2002), the body
temperature of some rocky shore invertebrates has been shown to vary by up to 25°C with
no harmful effects. These organisms’ body systems are well adapted to changing
conditions and they have evolved physiological mechanisms to tolerate a wide range of
body temperatures6.
Individual organisms can adjust their physiologies to short term changes over periods of
days or weeks by a process called acclimatisation. The properties of their body systems
may alter to resist the influence of the environmental change.
A good example of acclimatisation is a process called homeoviscous adaptation (‘homeo-
‘ = same; ‘viscous’ = opposite of fluid/runny) in cell membranes. As temperature
decreases, membranes become more viscous7, and as temperatures rise membranes
fluidity increases. Fluidity is an important physical property of a cell membrane and can
affect critical membrane functions. For instance, mitochondrial cell membranes are the
site of the majority of ATP production in aerobic respiration. Any changes to the fluidity
of mitochondrial membranes will alter their structure and reduce ATP production. The
effect on the organism is a reduction in respiratory rate and therefore metabolism.
6
These species are known as thermoconformers as their body temperature ‘conforms to’ and changes with
the environmental temperature.
7
Try this at home: observe how cooking oil becomes runnier (less viscous) in a frying pan as it gets hotter.
14 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
LT50 (°C)
Equator
Maximum vertical shore
position
Figure 9 Upper limits of thermal tolerance for intertidal crabs, Petrolisthes spp. Each symbol
represents a different species of the genus Petrolisthes i.e. each open circle is a species of crab
from California. LT50 is the temperature at which 50% of the crabs studied died during the
incubation experiment i.e. the average lethal limit. (Stillman, 2002).
In laboratory experiements, American intertidal species such as Californian blue mussels
(Mytilus californianus), abalone (Haliotis spp.8) and rock oysters (Crassostrea virginica)
can change the lipid content of their cell membranes to counter changes in its fluidity
caused by temperature changes (cited by Rais and Stillman, 2010); these intertidal
animals can respond by inserting more or less cholesterol and saturated fats into their cell
membranes which help the membranes to remain fluid or “runny” despite body
temperature changes.
Because intertidal organisms are ectothermic, they cannot regulate their body temperature
by physiological means. However, non-physiological adaptations have evolved in
intertidal organisms that help them to cope with extreme environmental temperatures by
reducing heat gain and promoting heat loss. Additionally, physiological adaptations have
evolved that protect or repair the cellular damage caused at extremely high body
temperatures.
A physiological adaptation to potentially damaging high environmental temperatures is
the production of heat shock proteins during periods of thermal stress. The proteins
function is to help maintain metabolic function by stabilising and repairing important
enzymes and protecting them from heat damage. These proteins have evolved in many
intertidal molluscs such as mussels, limpets, topshells and periwinkles to help them
tolerate high temperatures in the exposed upper shore.
8
New Zealand paua belong to the genus Haliotis which are known as abalone in Australia and the USA.
15 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
!
!
Figure 10 (Tomanek and Somero, 2000) shows the heat stress response of four different
species of closely related marine snails of the Tegula genus9.
Production of hsp70
Environmental temperature (°C)
Figure 10 Production of heat shock protein hsp70 marks the start
of the thermal stress zone in American subtidal and intertidal
topshells, Tegula spp. (Somero, 2000).
The snails were incubated at different temperatures and the amount of heat shock protein
(hsp70) in their body tissues was measured. T. rugosa is a subtropical species, whereas
the others are temperate species. Two species, T. funebralis and T. rugosa live in the
intertidal zone, whereas the other two species are subtidal. The function of hsp70 is to
repair and stabilise important metabolic enzymes in the snail’s tissues. It is only
produced at body temperatures that cause thermal stress. At the point where the
production of hsp70 falls to zero, the animal has exceeded its thermal tolerance and has
died.
The graph shows that the intertidal species experience thermal stress at higher
temperatures than the subtidal species; this implies that their tolerance range for
temperature is broader (above 12 °C at least). The graph also shows that the heat shock
response occurs over a much wider temperature range in the intertidal species. This
suggests that the temperature range of the stress zone of the intertidal species is wider
than that of the subtidal species i.e. they can manage a wider range of temperatures
through the production of the heat shock protein.
9
Tegula are found in North America are known as topshells. They are ecologically similar to our NZ
topshells, Diloma spp. and Nerita spp.
"B!Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
!
!
Heat gain can be minimised by having a relatively large body size compared to species
lower in the intertidal. This results in a low surface area to volume ratio and so the
surface available for heating is
kept relatively small. A
relatively large body size also
means that there is more body
tissue to heat up which therefore
takes longer for any significant
tempera-ture rise to happen.
This phenomenon can be seen
in periwinkles (Austrolittorina
sp.) and snakeskin chitons
(Sypharochiton pelliserpentis)
where larger individuals tend to
be distributed higher up the
shore as they can tolerate longer
exposure to higher temperatures
during low tide.
Dark colours absorb solar
radiation and so lead to heat !"#$%&'((''T(Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
Many shelled animals can tightly close their shell valves to create a high humidity
environment within their shells which helps to reduce water loss, for example many
marine snails, barnacles and mussels seal small amounts of seawater inside their shells
when uncovered by the falling tide. Soft bodied anemones retract their delicate tentacles
and withdraw inside their mucus-covered body, reducing their exposed surface area.
Behavioural adaptations in mobile intertidal animals to reduce water loss by evaporation
include by avoidance behaviour or seeking areas of high humidity. Many animals like
shore crabs and chitons bury themselves beneath loose sediments or cling to the moist
underside of rocks. Chitons respond to both light and gravity and tend to move
downward towards dark crevices when exposed to the air, behaviours known as
phototaxis and geotaxis.
Limpets scrape small body sized depressions in the rock (often on the shaded side) called
home scars which they can retreat to as the tide falls; they line the scars with mucus
which helps to seal them in and reduce water loss.
Topshells and periwinkles (Figure 12) cluster together in large numbers in crevices or on
the shaded side of rocks or boulders to increase local humidity and reduce the humidity
gradient, thereby reducing the rate of evaporation.
Figure 12 Blue-‐banded periwinkles (Austrolittorina antipodum) sheltering in crevices in the
rock on the upper shore during aerial exposure. The crevices create shaded microhabitats
with higher humidity than the surrounding rock surface which, together with the clumping
behaviour, and mucus pad secreted by the snail, reduces desiccation.
Intertidal organisms have also evolved structural adaptations to reduce water loss that
complement their behaviours. Purple shore crabs (Hemigrapsus sexdentatus) have a very
high tolerance to aerial exposure thanks to a waxy, waterproof covering on their carapace
18 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
called a chitinous cuticle. These crabs have a permeable section of their legs through
which they carry out gas exchange. This region is not covered by the waterproof cuticle
(which would prevent gas exchange) and so represents a potential source of water loss.
However, the crabs can tightly fold this portion of their legs up into their carapace and
maintain a high local humidity and reduce desiccation.
Most marine snails secrete watery mucus around their shell openings which forms a
protective layer of gelatinous slime, a physiological response to aerial exposure.
Periwinkles use this same mucus pad to attach to rocks when the tide is out10; they also
have very small gills that reduce the evaporative surface. Topshells, catseyes periwinkles
and whelks also have a thick, horny pad on their foot called an operculum that seals the
shell opening behind the snail.
Some organisms have evolved very high tolerances to desiccation stress. Periwinkles that
live in the highest levels of the shore can survive the loss of up to 70% of their water
content and are capable of rapidly re-hydrating when re-submerged by the rising tide.
Many types of seaweed are also capable of withstanding severe dehydration for short
periods and very rapid rehydration. The highest zoned seaweeds on the shore are very
tolerant of extreme dehydration, with species like sealettuce (Ulva sp.) and Neptune’s
Necklace (Hormosira banksii) capable of losing over 90% of their water content between
tidal cycles.
The rate of photosynthesis in seaweeds decreases as desiccation increases and, like
animals, many intertidal types of seaweed possess adaptations to reduce water loss. Many
seaweeds such as kelps secrete a slimy covering called mucilage11 which helps them
reduce water loss. The shape of the seaweeds fronds (‘leaves’) and the growth form of
the algae is also important. Like terrestrial cacti, the fleshy sacs of Neptune’s Necklace,
gummy weed (Splachnidium rugosum) and other sac-like algae store water so that
photosynthesis can continue; their rounded shape also provides a low surface area to
volume ratio so that evaporation rate is low. Coralline algae often grow in a thick turf
which helps to retain water and so keep the local humidity high.
1.7.2 Osmotic or salinity stress
Intertidal organisms experience stress due to changes in the salinity of their environment.
This stress is known as salinity or osmotic stress and can cause changes in the water or
salt content of their body tissues. If the salinity of the environment is greater than the
salinity of the body tissues of the organism, then the organism will tend to lose water by
osmosis and may gain salts by diffusion. If the salinity of the environment is lower than
the salinity of the body tissues, the organism would gain water and lose salts. Only when
the salinity of the body tissues became equal to the salinity of the environment would
there be no net change in water content. Changes in a cell’s water content create physical
stresses due to changes in cell volume and pressure which can lead to cell rupture or
collapse. Chemical stresses can also occur due to changes in the availability of water and
the concentration of reactants in the cell leading to reduced reaction rate or toxicity. If the
10
It also acts as an insulating pad to reduce heat transfer to and from the rock.
11
The mucilage of some brown seaweeds like kelp contains a sugar called agarose which binds water and is
extracted to make a agar gel.
19 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
organism does not counter these changes and regulate its water content (osmoregulation)
within tolerable levels it will die.
As the tide ebbs, surface films of seawater on the exposed shore become more saline as
freshwater evaporates. Likewise, freshwater evaporating from rock pools raises their
salinity; this is particularly significant in those smaller pools in the upper shore that are
exposed to the air for relatively long periods and lose large amounts of their freshwater by
evaporation. Conversely, large amounts of rainfall on the shore at low tide means that
organisms experience very low salinity conditions and freshwater can accumulate in rock
pools. Organisms in large rock pools will experience a slower decrease in salinity than
organisms on or under rocks that are washed with fresh rainwater.
Unlike most estuarine organisms, few intertidal organisms are well adapted to tolerate
changes in the salinity of their body tissues. Most intertidal species cannot control the salt
content of their body; these types of organisms are called osmoconformers.
The salinity of their tissues is generally similar to that of normal seawater (36 ppt.); after
all this is the environment that they evolved in and are adapted to. However, as for heat
stress, they can limit the effects of changing environmental salinities in their surroundings
by either tolerating, reducing or avoiding the effects. Mobile animals reduce the effects
of changing salinity by avoiding the stress and moving into a more suitable microhabitat.
Structural adaptations that reduce evaporation can provide protection against water loss
due to osmosis; for example the thick waterproof cuticles of crabs and barnacles are
effective barriers to water and salt movement. However, gas exchange structures like
gills are very vulnerable to osmotic stress. Gills need to have a large surface area and be
permeable to oxygen and carbon dioxide for efficient respiration. These structures are
kept moist and in almost continual contact with water. Whether water is trapped inside a
shell as in mussels and barnacles, is present as surface water coating the gills of crabs, or
is bathing the gills of fish in a rockpool, the salinity will change with increasing aerial
exposure. This can change the water and salt content of the organism’s tissues past its
tolerance limits.
Physiological adaptations to regulate the body fluid’s salt and water content help to
counter such changes and maintain a relatively steady salt and fluid balance through a
process called osmoregulation. Many invertebrates actively excrete salts like sodium and
chloride ions through their nephridia, gills, gut or antennal glands (Taylor and Andrew,
1988). Other organisms can reduce water movement by osmosis by matching the total
salinity of their internal fluids to the environment through exchanging ‘good’ ions and
dissolved substances for sodium and chloride ions e.g. seaweeds use glycerol, sucrose and
mannitol (Biebl, 1962); some invertebrates use amino acids. In this way, the organisms
can osmoregulate and so maintain a stable water content in their tissues.
Some organisms like periwinkles have simply evolved physiologies that are capable of
putting up with extreme salinity fluctuations and have a very wide passive tolerance
range. This is also true of many intertidal algae that have a greater physiological
tolerance range than their subtidal relatives.
1.7.3 Respiratory stress
20 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
With the exception of the high-shore pulmonate limpets12, most intertidal animals rely
directly on water for dissolved oxygen for aerobic respiration. Gas exchange structures
like gills need to be moist to function, therefore during aerial exposure, intertidal animals
need to maintain wet gill surfaces and avoid desiccation. Additionally, the concentration
of dissolved oxygen in water decreases with increasing temperature, so small rockpools
that experience the midday sun on the high shore are likely to contain far less oxygen than
larger, more shaded pools in the lower shore, especially if the low shore pools are near the
splash zone and aerated by breaking waves.
Periwinkles live at the very upper limit of the intertidal zone. Their gills are adapted to
efficiently extract oxygen from the air during exposure at low tide. Not only have their
gills decreased in size to reduce evaporative water loss, but the lining of their gill cavities
has a very rich blood supply that can exchange gases with the moist air surrounding the
mucus-lined shell opening, like a simple lung.
In terms of behavioural adaptations, avoidance of stressful conditions is common in
mobile animals like crabs and chitons that can move wit the tide and stay submerged.
Sessile barnacles and limpets are able to store air bubbles inside their gill cavities during
aerial exposure at low tide. The air acts as an oxygen reservoir and keeps the water
surrounding the gills enriched with oxygen. Topshells and whelks also trap water in the
jelly-like mucus around the edge of their gill-bearing mantle cavities. Oxygen from the
air dissolves into this moist layer and can then be extracted by the gills. However, even
allowing for the limited ability of some intertidal organisms to extract some oxygen from
the air during exposure, most of these marine animals cannot maintain a high respiratory
demand when the tide is out. Their gas exchange organs are mainly adapted for
extracting oxygen from seawater; prolonged activity during exposure will rapidly deplete
oxygen in their tissues and carbon dioxide will accumulate, lowering cellular pH. The
most common physiological adaptation is to reduce the oxygen demand and rate of
carbon dioxide production by lowering the metabolic (and therefore respiration) rate, and
fuelling essential life processes by anaerobic respiration; the lactic acid build up is then
reversed during re-submergence as the tide rises.
12
This group of high-shore molluscs are not thought to be closely related to the common shore limpets, but
have evolved from a relatively recent terrestrial ancestor like the land snails. They possess a pseudo-lung
rather than gills that enables them to carry out gas exchange in the air and have adapted to the salty
conditions on the upper shore. Their resemblance to the true marine limpets is a neat example of
convergent evolution: where distantly related species have been ‘shaped’ by natural selection in the same
way, by the same environmental pressures in their similar niches.
21 Ecology of the New Zealand Rocky Shore Community:
A Resource for NCEA Level 2 Biology
PART 2: SURVEYING THE ROCKY SHORE
2.1 INTRODUCTION: WHY STUDY THE ROCKY INTERTIDAL SHORE?
New Zealand is a coastal nation. It has one of the largest coastlines relative to our land
area in the world. Our coast is under pressure from many different users. The intertidal
border between the sea (marine) and land (terrestrial) environments are susceptible to
impacts from both the land and the sea including pollution; overfishing; mineral
extraction and dredging; invasive pest species like Undaria seaweed; nutrient enrichment
and increased sediment in rivers from run-off; habitat loss and alteration; and the global
effects of climate change that include rising sea level, increased air and sea temperatures,
disrupted weather systems and ocean acidification.
The intertidal border between the land and the sea is a productive and diverse habitat. It
has a high biodiversity and is home to many slow-moving or sessile species that are
relatively easily quantified for monitoring changes. Collecting data on species
distribution, abundance, community structure and biodiversity will provide a useful
baseline for detecting any future changes.
2.2 HOW TO CARRY OUT THE SHORE SURVEY
The aim for the class will be to construct a table that shows the mean density and
distribution of intertidal organisms from the upper limit of the tide’s influence to the low
water mark.
A class will set up 3-5 randomly placed transects that will run from the upper limit of the
high tide’s influence (the splash zone) down the shore to the water’s edge at low tide.
Each group of 3-4 people will be responsible for surveying one transect.
Along each transect there will be 11 sampling points. At these points a quadrat will be
placed to measure the abundance of all13 of the animals and plants found there. The
sampling points will be evenly spaced along the transect at regular intervals of 1/10 the
total transect length. Each sampling point will correspond to a different tidal level i.e.
quadrat 1 (Q1) will always be at the upper limit of the high shore; Q5 will be the mid
shore sample, and Q11 the lowest of the low shore samples (Figure 13).
As each group will use the same method, their data can be pooled. At the end of the
survey the pooled data can be used to calculate the mean density at each shore level for all
species found in the survey.
Using a standardised method means that results from different times and places can be
compared.
13
You should not assume that you know what the dominant or important species in an intertidal community
are before you survey it and choose to count only those species; a valid method for a community survey
would collect data on all species that can then be analysed for patterns that might suggest significant
interactions between different species.
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